This application relates to a method for inhibiting eukaryotic protein kinases using lichen-derived organic acids such as vulpinic acid and usnic acid.
Kinase and phosphatase enzymes play important roles in the regulation of both eukaryotic and prokaryotic cells. For example, in eukaryotic cells, the control of proliferation and differentiation is achieved by multiple signal transduction pathways that are regulated by the coordinated action of protein kinases and phosphatases.
Kinase activity in eukaryotes can be classified as one of three types: those enzymes which phosphorylate tyrosine residues; those which are specific for serine or threonine residues; and those which have dual specificity for both tyrosine and serine/threonine residues. Because of the importance of these enzymes in eukaryotic regulatory processes, it would be highly desirable to be able to inhibit kinases of the various classes selectively to assist in the elucidation of kinase and phosphatase mediated pathways, particularly those that may be of medical significance. In addition, selective kinase or phosphatase inhibitors have potential uses as therapeutics. For example, it has been reported that a tyrosine kinase blocker designated AG-940 specifically inhibits the Jak-2 protein tyrosine kinase which is deregulated and constitutively activated in the leukemic cells of acute lymphoblastic leukemia (ALL) patients. Meydan, et al. xe2x80x9cInhibition of acute lymphoblastic leukaemia by a Jak-2 inhibitorxe2x80x9d, Nature 379: 645-648 (1996). This inhibition induced changes in cells consistent with entry into apoptosis when tested in vitro. Further, the intravenous administration of the inhibitor into mice previously injected with ALL cells has been shown to be effective to eradicate the ALL cells from the marrow.
Notwithstanding the potential uses of kinase and phosphatase inhibitors, the number of known and characterized inhibitors is quite small. Staurosporine and K-252a are known to act as generalized kinase inhibitors, while herbimycin and radicol specifically inhibit tyrosine kinases, albeit with fairly-low effectiveness. There are no known specific inhibitors for the MAP kinase family, an important group of enzymes thought to be central in the transmission of a wide variety of signals received at the cellular membrane to the transcriptional and replication machinery of the nucleus.
To facilitate the identification of new kinase and phosphatase inhibitors, we have developed an assay which is described in our prior U.S. Pat. No. 5,770,392, issued Jun. 23, 1998 and in a PCT Publication No. WO98/17822 claiming priority therefrom, which are incorporated herein by reference. This assay basically involves the steps of:
adding the material to be tested for kinase inhibitory activity to a growing culture of a prokaryotic organism such as a streptomycete;
allowing the culture to grow for a period of time in the presence of the material; and
observing the culture for altered development relative to development of the prokaryotic organism grown in the absence of the material. Observation of altered development is indicative that the material has activity as an inhibitor of post-translational protein phosphorylation. In particular, the material to be tested can be added to a growing culture of the prokaryotic organism by placing a carrier disk bearing the material on a freshly seeded plate. Inhibition of the development of aerial mycelia and spore formation is an indicator that the material has activity as an inhibitor of post-translational protein phosphorylation.
Using the assay method of our prior invention, we have screened a number of lichen extracts. Surprisingly, the majority of these extracts exhibited the ability to inhibit sporulation of our preferred tester strain Streptomyces 85E, suggesting that there might be a few common inhibitors produced in a wide variety of lichens. Following this line of reasoning, it was discovered that lichen-derived organic acids such as vulpinic acid and usnic acid are effective inhibitors of eukaryotic protein kinase activity. Thus, the present invention provides a method for inhibiting eukaryotic protein kinase activity present in a sample or organism comprising the step of adding to the sample or organism an effective inhibitory amount of a lichen-derived organic acid.